Jude 21 reads, ' Keep yourselves in God’s love as you wait for the mercy of our Lord Jesus Christ to bring you to eternal life.' Again we are called here to take responsibility for our own faith. Remember that Jude's readers are facing false teachers and he has dished out heinous warnings to these teachers and their followers of a terrible fate. Here, the readers are to resist and keep themselves in God's love. 'Keep' is tereo and has the sense of 'guard oneself'; that is, watch your own faith. I am not one who agrees with the 'once-saved-always-saved' theology. I think such a view has to do exegetical gymnastics with a raft of texts which suggest one can initially believe, but do so in vain through falling away (e.g. 1 Cor 15:1-2; Heb warning passages). Hence, I believe Jude is warning his readers, look after yourself in the faith.
'In God's love' here is the language of relationship. Walk in faithful obedience and you remain in relationship with God. His love is there for all of us who seek to remain in that relationship. This is the language of intention not perfection, seeking to obey and honour and love God despite failing at times. It is the only place in the universe to be isn't it, in God's love.
'As you wait for the mercy of our Lord Jesus Christ to bring you to eternal life' is an example of the 'already-not yet' eschatology. This refers to the second coming at which time Jesus will return and take us to eternal life. Where eternal life will be lived out is ambiguous, perhaps 'in heaven', 'perhaps on a restored earth', perhaps in a merge of both in some way. 'The mercy' calls to mind the basis of salvation, Christ's mercy. As we live in obedience, this is our future hope. Where Christ through his salvation by his death and resurrection, save us despite our sin.
This is a great verse. It calls us to take responsibility for our faith, to live it out in obedience, stop blaming anyone else or expecting anyone else to do so. It tells us how to live and tells us what we live for. We live for that day when Jesus returns to live eternally with us. Thanks God for your love, thanks for your mercy. Help me to keep myself in your love!